The present invention is directed to an optical scanning arrangement and a method for reading a code.
Scanning arrangements with scanning devices for the reading of codes are known (DE-PS 23 40 688). In general these scanning devices operate in such a way that via an optical system and a mirror wheel or an oscillating mirror a laser beam forming a scanning beam periodically scans the surface on which the code is arranged. Depending on whether the scanning light bead falls onto a lighter or darker point of a code more or less light is scattered backwards to the scanning device which detects this quantity of light, for example in accordance with the autocollimation principle, and transmits a corresponding evaluation signal.
Decisive for the reliability of code reading is now the sharpness and size of the scanning light bead which is guided over the code. The sharpness of the scanning light bead is thereby materially co-determined by the image forming characteristics of the scanning device. The resolution of fine image structures is restricted by the scanning frequency of the reflected light and the light bead diameter.
The light bead shape and size changes in dependence on the spacing of the scanning device from the surface carrying the code (a follow-up adjustment of the sharpness and/or enlargement does not take place via an autofocussing device). The modulation of the light scattered back from the code only corresponds in a narrowly restricted range of distances in a defined and evaluatable manner to the actual sequence of code fields. Outside of this range of focus, the light bead shape and size change such that the light dark modulation of the light scattered back from the code no longer reproduces the relationships of the original code sufficiently accurately in order to generate a corresponding output signal.
Complicated image distortions arise which in particular present a problem in the following applications.
Recognition and classification of images by comparison with normal images; PA1 strongly fluctuating changes of the spacing of the scanning device from the surface carrying the code, with the limits of the range of focus being exceeded; PA1 rapid image change which makes it very difficult to autofocus regulation by optical means.
In accordance with the prior art the listed difficulties are typically overcome with expensive image forming optical systems or with a very rapidly operating autofocus regulation system which is however very complicated and costly for this purpose.